For example if you have a data refresh file with Columns that looks like this:
                       Gender                        Prefix
Jane Smith      F                                 Ms.

John Smith 

Then any user that has nothing in the Columns, such as John Smith's gender and prefix would be overwritten with a blank.

If you have a  Data Refresh File with Columns that looks like this:
 

                       Gender                        Prefix                              Birth Date
Jane Smith      F                             Ms.

John Smith      M                            Mr.

Then those whole blank Columns will just be omitted in the Data Refresh and that Data would remain unchanged. Such as Jane and John's Birth Dates would not be changed.

Note: Usernames do not get overwritten by blank fields. Will data refresh change or delete an existing user's login credentials? Furthermore, a data refresh does not overwrite an individual user’s address, e-mails, phones, or occupation data in Core where blank (NULL) in the data refresh file, but instead marks them for possible deletion. Since an address and occupation comprise multiple fields, all fields (for an individual user) must be blank in the file to signal the refresh to mark them. There are three data refresh procedures available for overwriting this data in Core with blanks: e-mails, cc e-mails, and one encompassing addresses, phones, and occupations.  A school can ask Data to run any of the procedures for a single refresh, or automatically every time the refresh is performed.